That means expressions with only literals or enumeration values that result in a single integer value, such as case FOO + 1:.

The expression in switch can be (almost) whatever you want, but anything after case needs to be a single constant integer (or something that trivially results in that). So, no comparisons, no boolean logic, no function calls, etc. -- integers, like 5 or 8, or 7123, or 213+5.